1. Field of the Invention
The present application relates to an optical amplifying unit and a method to control the optical amplifying unit.
2. Background Arts
An optical communication system often installs optical amplifying units each implementing a semiconductor optical amplifier (SOA) between an optical transmitter and an optical receiver. Such an optical amplifying unit first detects power of an incident optical beam, then, amplifies the incident optical beam such that power of an output beam is maintained in constant, which is often called as the automatic power control (APC). Because the incident optical beam entering the amplifying unit shows various optical power depending on types of optical transmitters, optical losses in transmission lines, and so on. Accordingly, the amplifying unit often attenuates the power of the incident optical beam, then, amplifies the attenuated beam.
Japanese Patent laid open No. H09-224016A has disclosed an optical amplifying unit that provides optical attenuators to attenuate incident beams each coming from optical transmitters, an optical multiplexer to multiplex attenuated optical beams, and an optical amplifier to amplify the multiplexed optical beam. The amplifying unit disclosed therein feeds the power of the output beam output from the optical amplifier back to the optical attenuators but does not detect the power of the incident beams. When an amplifying unit provides a photodetector to sense power of the incident beam to operate the optical amplifier set in downstream of the attenuator, the amplifying unit inherently becomes complex and enlarges a size thereof. In particular, when the amplifying unit receives a plurality of incident beams like that disclosed in the prior art, the number of the photodetectors set in the front end of the attenuator becomes large and the amplifying unit becomes further complex.